Twinkle twinkle little star…

Stars seem to twinkle because we see them through th layers of the Earth’s atmosphere. As light passes through these layers, it is distorted so that the amount we actually see changes constantly. The stars nearest the horizon appear to twinkle the most because the light is passing through a greater depth of atmosphere. Stars do not twinkle when viewed depth of atmosphere. Starts do not twinkle when viewed from space, which is why telescope in space, such as the Hubble, give the best possible view of distant stars and galaxies.

Star facts
A star is a luminous body of gas, mostly hydrogen and helium. Stars generate light, which makes it possible for us to see them with a telescope or the naked eye. They also release energy from nuclear fission reactions in their core.

Brightest
Not counting the Sun, the brightest star as seen from Earth is Sirius, known as the dog star, in the constellation of Canis Major. It has a diameter of 149,598,020km and is more than 24 times brighter than the Sun. The star Cygnus OB2 No 12, discovered in 1993, is so far away that it cannot been seen from Earth. It may be the brightest star in the galaxy – up to six million times as bright as our Sun.

Heaviest
HDE 2698 10 is a star in the Large Magellanic Cloud – 170,000 light years from Earth (a light year is the distance light travels in a year). It has been discovered by the Hopkins Ultraviolet Telescope to be 190 times as heavy as our Sun.

Largest
The largest star is the M-class supergiant Betelgeuse, or Alpha Orionis. It is the top left star in the constellation of Orion, which is 310 light years away. It has a diameter of 700 million km, which is about 500 times greater than that of the sun.

Nearest
Proxima Centauri, discovered in 1915, is 4.22 light years (39,953,525,879,212km) from Earth. A spaceship moving at 40,000km/h – which is faster than any human has yet traveled in space – would take more than 114,000 years to reach it.

Supernovae
These are vast explosions in which a whole star is blown up. They are extremely bright, rivaling for a few days the combined light output of all the stars in the galaxy. Supernovae are rate – the last one in our galaxy was seen in 1604 by the German astronomer Johannes Kepler.

Quasars
Quasars are extremely distant radio galaxies – galaxies giving out large amounts of radio energy – and the brightest objects in the Universe. Even those near the most distant edge of the observable Universe are easily detected by small radio telescopes. Their radio emission is typically 1,000,000 to 100,000,000 times greater than that of a normal galaxy, and they are as bright as or brighter than the brightest radiogalaxies.

Black holes
A black hole is a star that has collapsed into itself. It has a surface gravity so powerful that nothing can escape from within it.

Time tellers

The following are some landmarks in the history of telling the time.
1500-1300BC
Sundials are used in Egypt: as the Earth rotates, the gnomon - the upright part of the sundial - casts a shadow which moves to indicate the time.

c 400 BC
Water clocks are used in Greece: as water drains from a container, each level it reaches represents a period of time.

c 890
In England people use candles marked with time intervals.

12th century
The hourglass, familiar to us as an eggtimer, is used by monks to show times of prayer.

1325
The first clock with a dial is installed in Norwich Cathedral, England.

1335
The first clock to strike the hours is made in Milan, Italy.

1350
The oldest known surviving alarm clock is made in Wurzburg, Germany.

1364
Clocks are first used in people's homes.

1386
Salisbury Cathedral's clock is installed. This is the world's oldest clock in working order.

1462
The earliest descrition of a watch is written in Italy.

1641
The idea of the pendulum clock is proposed by Vincenzio Galilei, son of the famous astronomer Galileo.

1657
The first pendulum clocks are made in Holland.

c 1665
The first watches with minute and second hands are made.

1759
John Harrison's marine chronometer is made. Accurate timekeeping at sea is important for calculating position, but previously the rolling of a ship had made it impossible.

1880
Greenwich Mean Time becomes the standard from which time around the world is set.

1880
The first practical wristwatches are made for the German navy.

1928
The first quartz crystal clock is made.

1949
The first atomic clock is built.

1957
The first battery watches are marketed in the USA.

1969
Quartz wristwatches are first sold in Japan.

1970
Digital wathces and displays become widely used and can be made and sold cheaply.

2006
40 radio clocks around the world transmit signals that enable people to set their clocks and watches accurately.